An electrical drive unit includes a number of power generating components like transistors. Power transistors generate, apart from power, a lot of heat that needs to be transferred from the components to a heat sink. The best heat-conducting transistors have a metal surface on one side, the inside of which metal surface is connected to the transistor component, and in most cases the transistor component thus has to be electrically isolated from the heat sink. Such isolation is normally achieved by a thin ceramic substrate of aluminum oxide or some heat conductive rubber positioned between the metal surface and the heat sink.
Traditionally these transistors are mounted in an upright position along an edge of a printed circuit board (PCB) and are clamped to a heat sink. One problem with this method is that you have to place the transistors close to the edge of the PCB even if they electrically would be better placed in other portions of the PCB, such as the centre of the PCB. Another problem is that the PCB area has to be decreased, for a predetermined assembly size, if the heat sink is situated under the PCB, as one part of the heat sink has to protrude up in order to collect heat from the transistors and transport it down to the heat sink. Turning the board upside down diminishes this problem, but then the clamping of the transistors gets more complicated instead. Soldering the transistors in an upright position can also be difficult and requires soldering fixtures.
One known method for cooling of transistors is to place the transistors between the PCB and the heat sink and with a spring exert a pressure on the opposite side of the PCB in order to clamp the transistor against the heat sink, optionally with an electrical isolation between the transistors and the heat sink. The PCB is sometimes cut between the components to make it more flexible and ensure a good individual pressure for each component. A problem with this solution is that it takes up a large portion of the PCB area on both sides thereof, since both the transistors and the spring push against the PCB.
Another known way to cool transistors is to use a power module, a component consisting of a number of chip-bonded transistors. These power modules have one surface that is heat conductive and already electrically isolated. They often come in two parts where one is placed between the PCB and the heat sink and on the other side of the PCB is a lid placed that is providing a pressure towards the heat sink with the help of one or more screws. A problem with such power modules is that they are expensive and allocate a large PCB area.
A third known way to cool transistors is to use surface mounted power transistors on an additional special PCB built up with a thick aluminum layer at the bottom, which is quite an expensive solution. The transistors are then soldered so that the generated heat easily flows down through the board to a heat sink. The fact that you need two boards with power connections in between them increases production costs and makes the quality more difficult to control.